Photoresist is a light sensitive material used in certain fabrication processes to form a patterned coating on a work piece, e.g., a semiconductor wafer, during processing. After exposing the photoresist coated surface to a pattern of high energy radiation, a portion of the photoresist is removed to reveal the surface below, leaving the rest of the surface protected. Semiconductor processes such as deposition, etching, and other processes may then be performed on the partially uncovered surface and remaining photoresist. After performing one or more semiconductor processes, the remaining photoresist can be removed in a strip operation.
Both negative and positive photoresists may be used to pattern a wafer. With positive photoresist, exposure to the high energy radiation causes the resist material to become more soluble in the developer, as compared to parts of the resist that are not exposed to the radiation. As such, when a substrate patterned with positive photoresist is exposed to developer, the areas where the resist was exposed to radiation are removed, while the resist in non-exposed areas remains intact. Negative photoresists work in the opposite manner, in that exposure of a negative photoresist to high energy radiation causes the negative photoresist material to become less soluble in developer. Therefore, after contact with the developer, the areas of the negative resist that were exposed to radiation remain intact, while areas of the resist that were not exposed are removed.
One area in which negative photoresists have been adopted for use is in Wafer Level Packaging (WLP) processes such as in bump and copper pillar applications. For example, these processes may utilize certain negative dry film and spin-on photoresists. These materials have proven especially advantageous in these applications because they have good adhesion to substrates, excellent compatibility with plating chemistries, and result in well-formed, vertical profiles with essentially no footing at the feature base, even for deep features (e.g., features deeper than about 20 μm). Negative photoresists present certain fabrication problems, however. One main issue with the use of these materials is that negative photoresists are quite difficult to remove.